Abstract The ability of the immune system to recognize and initiate an immune response to foreign particles such as bacteria and viruses has generated interest in development of particulate vaccine technologies. We are developing an innovative approach to produce particulate vaccines via layer-by-layer (LbL) fabrication of synthetic microparticles (MP). We have shown that LbL-MP loaded with epitopes from respiratory syncytial virus (RSV) or Plasmodium falciparum (Pf) elicit potent and balanced humoral and cellular immune responses that protect the immunized host from infection and disease following pathogen challenge. The LbL-MP vaccines are immunogenic and efficacious when administered at low doses (1-10 g) in aqueous suspension without any exogenous adjuvant, thus offering safety advantages over competing vaccine technologies. In this Phase I feasibility study, we will improve the LbL-MP technology by loading the MP into microneedle arrays designed to deliver the payload directly into the skin, a rich reservoir of immune cells including Langerhans cells and dermal dendritic cells. We will use two LbL-MP that we have already extensively characterized, one loaded with two epitopes from RSV and one loaded with three epitopes from Pf, the causative agent of human malaria. Each MP will be loaded into microneedle patches using our standard patch formulation which includes trehalose for vaccine stability. The efficiency of MP loading will be monitored by dissolution of the patch in saline buffer and amino acid analysis to determine the amount of vaccine antigen recovered, while integrity of the recovered vaccine antigen will be monitored by ELISA using monoclonal antibody specific for the target antigen. We will use mouse models to compare the immunogenicity of MP formulated in microneedle patches against that of parenteral immunization with the same MP in aqueous suspension without adjuvant. In the RSV model, we will examine efficacy of the microneedle patches by challenging the immunized mice with live RSV and monitoring lung viral burden post-challenge. We anticipate that microneedle patch delivery of MP will result in greater immune potency (e.g., higher antibody titer elicited or lower dose required), more favorable immune phenotype (e.g., isotype switching and Th1- biased cellular response), and superior efficacy (reduction in viral burden post-challenge) when compared to parenteral injection of the same LbL-MP. This advancement in the LbL-MP vaccine platform should lead to improved vaccine potency, easier administration, and greater patient acceptance due to its minimally invasive nature.